Compare dropdown fails when selecting branch with no common merge-base (#37470) (#37472)

## Summary

- handle compare requests where base and head refs have no common merge
base without returning 500
- keep the compare branch selectors usable and show a clear warning
message
- add regression coverage for unrelated-history compare selection and
merge-base error detection

Fixes #37469 



Manuel Backport of: https://github.com/go-gitea/gitea/pull/37470

---------

Co-authored-by: Codex <codex@openai.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/gitrepo/merge.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/git/gitcmd"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // MergeBase checks and returns merge base of two commits.
@@ -16,6 +17,9 @@ func MergeBase(ctx context.Context, repo Repository, baseCommitID, headCommitID
 	mergeBase, _, err := RunCmdString(ctx, repo, gitcmd.NewCommand("merge-base").
 		AddDashesAndList(baseCommitID, headCommitID))
 	if err != nil {
+		if gitcmd.IsErrorExitCode(err, 1) {
+			return "", util.NewNotExistErrorf("get merge-base of %s and %s failed", baseCommitID, headCommitID)
+		}
 		return "", fmt.Errorf("get merge-base of %s and %s failed: %w", baseCommitID, headCommitID, err)
 	}
 	return strings.TrimSpace(mergeBase), nil